I have a page that has a list of links on the left side and when you mouse over them, the contents appear on the right side. I put the contents in a fixed position using CSS so that when you scroll down the list of links (there is a scroll bar on the right side of the page), the content stays in place.
I did this because the content would remain at the top of the page while you scrolled down the list of links and eventually was not visible. Using the CSS position:fixed, the content stays in that fixed position relative to the browser. But when you scroll down the web browser, the content floats over or eventually out of the box still in it's fixed position from the browser.